1. Field of the Invention
The present invention relates to a process for the manufacture of low density, i.e. 1 to 2 pounds per cubic foot, flexible polyurethane foam. More particularly, the instant invention involves the catalyzed reaction of: (1) high reactivity, high ethylene oxide, high functionality polyols or polyahls; (2) hydrophilic polyhydric compounds, (3) water above 4.0 parts per hundred parts, and (4) organic polyisocyanates to obtain flexible polyurethane foam with low density.
2. Description of the Prior Art
High resilience (HR) foam technology has provided improved physical foam properties and processing advantages compared with the conventional technology of foam production. The HR-foams generally possess outstanding resiliency and cushioning qualities.
The HR foam properties and processing advantages are considered to be related to the employment of highly reactive polyether or polymer polyols with more than 60 percent of primary hydroxyl groups and molecular weights from 4800 to 6500.
In the manufacture of HR foam, the production of very low density parts (below 1.5 pcf) has been very difficult to achieve. Density reduction is achieved by either increasing the water level and thus carbon dioxide evolution or by the use of chlorofluorocarbons. Because of environmental concerns with the chlorofluorocarbons, the former approach is preferred. As the water level is increased, the exotherm increases creating a large temperature gradient between the foam core and the mold surface using a conventional amine gellation catalyst. Typically as the water levels in molded foam systems has exceeded 4.0 parts per one hundred parts of polyol in a formulation it has become more difficult to make good parts. Careful control of molding conditions and certain delayed action amine catalysts have permitted some commercial systems to run at 4.5 to 4.9 water levels but higher levels have been unattainable. In order to obtain sufficient gas generation to achieve these low densities with a high water system, the level of water and isocyanate are quite high 65-75 parts of isocyanate per 100 parts of polyol for a 1.2 pcf foam (6.5 pphp water) vs 45-50 pphp for 1.8-2.0 pcf foam (4.2 pphp water). This difference results in very unstable processing conditions. The water/isocyanate (blowing) reaction generates a large amount of heat. The heat significantly increases the rate of reaction. This means that all of the reactions take place faster than they do in most current commercial systems. In particular, the gas evolution occurs so quickly and violently that it is very difficult to balance the polyol/isocyanate (gellation) reaction so that the gases can be trapped to make a foam. The net result has been shear collapse and instability around mold venting areas and in restricted areas in the mold (i.e. around fences or inserts in the mold).
The problem facing the industry was to bring the blowing reaction under control so it could be properly balanced with the gellation reaction so that foam could be formed under reasonable conditions thereby obtaining good processibility.
In addition, the coming-of-age of rapid demold systems has created a need for improved systems. Rapid demold systems operated at a 3 minute demold. At times when the carousel is not being fully utilized the demold times can be accelerated to 2.5 minutes. However, typically an indexing carousel line has 2 or 3 stations that are not utilized between the pour station and the demold station. This means that the productivity could be increased if a foam system could be developed which was cured at 1.5-2.0 minutes and still retained its ability to flow well in the mold.
A paper published by Kollmeier, et al. (Th. Goldschmidt Ag.), had described the use of glycerine in urethane foams resulting in delayed cream time.
In their article, Kollmeier, et al. refer to highly reactive polyols having more than 60% primary hydroxyl groups. However, it has now been found that the best foams are made with more highly reactive polyols (75-90+% primary hydroxyl groups). While it is possible to pour acceptable foams with lower reactive polyols, it requires more glycerine and the systems are less flexible to formulate since additional glycerine means even slower cream times. Furthermore, at high pphp water levels these products give coarse cells.